Recently, thermoradiative (TR) cells have been proposed as heat engines to convert heat into electricity. The simplest form of a TR cell consists of a p-n junction that is heated to a higher temperature Tc than ambient Ta. The concept was demonstrated by experiments, although the realized efficiency was low. It is possible to boost the efficiency by designing the TR cell with nanophotonic approaches, which have been widely explored for photovoltaic (PV) and thermophotovoltaic (TPV) cells, such that selective radiation at a narrow band just above the bandgap energy is achieved. However, in this case the radiation power would be greatly suppressed. As a result, the generated power density would be extremely small.
However, with near-field resonant coupling, the radiation can go beyond the blackbody limit, and all the radiation power can be “squeezed” into a narrow bandwidth around the resonance. Based on this understanding, a heat sink was placed in close vicinity of the TR cell. It was shown that by near-field coupling of the photons generated from the TR cell to the phonon polariton mode that is supported on the surface of the heat sink (whose dispersion is described by a Lorentz model), both the conversion efficiency and the generated power density can be greatly enhanced when the resonance is very close to the bandgap energy of the TR cell. The near-field enhancement effect of TR cells was further explored, and it was shown that a metallic material, whose dispersion is described by a Drude model and supports surface plasmon polaritons (SPPs), is also good candidate for heat sink, and can have an even more significant output power density enhancement effect as compared with Lorentz type materials. The enhancement effect was understood from the impedance matching condition derived from coupled-mode theory. In the case of radiative energy transfer dominated by resonant coupling between two objects (TR cell and the heat sink, in the case of TR device), the transfer is maximized when the resonance decays into the two objects at the same rate. This condition is easier to achieve with a Drude type metallic material. An additional advantage with metals as heat sink is their typically larger thermal conductivities compared with insulators. The faster heat dissipation makes it easier to maintain a temperature close to the ambient.
To use TR cell based devices to harvest low-grade waste heat with temperature of 1000 K or lower, the preferred band gap energy of TR cell is 0.3 eV or lower. In order for the near-field resonant coupling to work, the resonant mode needs to have an energy slightly above the band gap energy of the cell. However, typical noble metals have surface plasmon resonance with much higher energy. For example, plasma frequency ωp of gold is around 9 eV, and the frequency of SPP on planar gold surface ωSPP=ωp/√{square root over (2)} is around 6.4 eV, which is more than 20 times higher than the typical TR cell band gap energy.
The large mismatch between the bandgap frequency of the TR cell and the SPP resonant frequency of flat metal surfaces makes the TR device very inefficient. Modifications of the device design are needed to improve the power generation performance.